1. Field of the Invention
The present invention relates to a device and method for protecting a system against lightning, and more particularly to a device for protecting a wireless communication system from impulse surges occurring in the system under the impact of lightning discharges.
2. Description of the Related Art
A general wireless communication system includes a mobile switching center (MSC), a plurality of base station controllers (BSCs) connected downstream of the mobile switching center, and a plurality of base transceiver stations (BTSs) connected downstream of each base station controller. In the wireless communication system, the mobile switching center and the base station controller are, in most cases, positioned inside a building. It is possible to protect the system from impulse surges using a lightning discharge protection device located in the building, which employs elements for preventing impulse voltage of a relatively low electric field that may occur in cables.
On the other hand, the base transceiver stations (hereinafter referred to as “base stations”) connected downstream of a base station controller are in most cases installed outside the building in order to communicate with wireless terminals. When the base stations are installed outside a building in this manner, it is essential to protect them against lightning because the base stations have a wireless antenna and thus very weak resistance to lightning. In other words, if lightning occurs, it may induce a transient high-voltage current through the base station's antenna, which is highly likely to damage the base station system because the base station system is composed of semiconductor elements. For this reason, various surge protection devices have been developed to protect the base station devices from lighting.
An arrestor is generally used as the surge protection device, which is classified into the following four types. The first type is an arrestor using a high pass filter, the second is an arrester using a gas capsule, the third is a λ/4 shorting stub arrestor, and the fourth is an arrestor using a semiconductor Transient Voltage Suppressor (TVS). These arrestors have the following problems.
First, the arrestor using the high pass filter has a problem in that it has a high residual pulse level due to a high inductance value. In other words, the arrestor's inductance provides a very high resistance against high frequency signals, but a residual pulse occurs after the high frequency signals are input.
Second, the arrestor using the gas capsule does not operate for surges having a voltage lower than a dynamic spark-over voltage. However, the dynamic spark-over voltage is a very high voltage of 900V in general. Since the dynamic spark-over voltage is set very high, this arrestor does not operate for overvoltages lower than the spark-over voltage, for example, 500V or 600V. This arrestor thus has a problem in that, when adapted for a base station composed of semiconductor elements, it cannot protect the system from the non-activating overvoltages.
Third, the λ/4 shorting stub arrestor has excellent performance in terms of all characteristics. However, it is difficult to use this arrestor in a base station system since its stub is short-circuited to the ground. Specifically, a DC current must be supplied through an antenna feeder line because the base station system operates while employing amplifiers next to an antenna provided in the system. In other words, since a power amplifier for transmission and a low noise amplifier (LNA) for reception are positioned next to the antenna, it is required to supply DC current. However, since the stub is short-circuited to the ground, the resistance of the stub and the ground is very low, making it difficult to supply the DC current.
Fourth, the arrestor using the semiconductor TVS has no resistance to high currents since it uses the semiconductor element. Thus, it is practically impossible for this arrestor to protect a system from a lightning strike if the lightning current directly enters the system.
One example of the above devices will now be described with reference to FIG. 1. FIG. 1 shows a prior art device disclosed in U.S. Pat. No. 5,978,199 entitled “EMP-Charge-Eliminator”, issued on November 1999, which is incorporated herein by reference. This device will also be referred to as a “prototype”.
As shown in FIG. 1, the device includes a high frequency line 3 that connects input and output terminals 1 and 2. In addition, a decoupling filter 4 and a gas arrestor 5 are connected in series between the high frequency line 3 and the ground. The decoupling filter 4 include λ/4 lines, where λ is the central passband wavelength thereof.
The device will now be described with reference to FIG. 1. If a surge impulse having an amplitude reaching a response voltage of the arrester 5 is input to the input terminal 1, the impulse voltage signal flows to the arrestor 5 through the decoupling filter 4. As input to the arrestor 5, the impulse voltage becomes an effecting impulse, which then flows into the ground. In this manner, the device prevents overvoltage signals from flowing into the circuit when an overvoltage impulse occurs. On the other hand, if there is no overvoltage, the impact of the arrestor 5 on the high frequency line 3 is neutralized by the decoupling filter 4 composed of several λ/4 sections. The circuit shown in FIG. 1 enables operation of equipment in any frequency range up to 18 GHz.
The gas arrestor 5 in the circuit shown in FIG. 1 can be used in the frequency range below 2 GHz, but cannot limit voltage surges below 100–200 V. The circuit thus has a problem in that it cannot protect the semiconductor antenna amplifiers.